1. Technical Field
The present invention relates, generally, to methods and apparatus for selectively demetallizing a web having a metallic surface, and more particularly, to patterned removal of metal from a metallized traveling web.
2. Background Art and Technical Problems
In certain applications it is desirable to demetallize predetermined portions of a metallized substrate. For example, in the packaging of microwavable foodstuffs, certain interior portions of the package are advantageously metallized to facilitate browning and crisping of the food; the metallized paperboard absorbs microwave energy and radiates heat in excess of the heat typically produced by the food alone. More particularly, in packages containing a combination of different foods, it is often desirable to brown only certain of the foods, the remaining foods being heated solely through vibration of the food molecules. Accordingly, packaging material having predetermined selected metallized and demetallized regions is desirable. Such selective demetallization of microwave packaging has typically been effected using masks and chemical etchants.
In general, techniques for electrically demetallizing portions of metallized webs are known. For example, Heywang et al. U.S. Pat. No. 3,895,207, issued July 15, 1975, discloses a method and apparatus for the production of metal-free paths on metallized insulator foils for use in the production of capacitors by winding alternating strips of metallized paper and dielectric film about a common spool. A moving flexible web having a metallized surface is directed along a path, the web being partially wrapped around a grounded conductive roller. A metal tape (wire) serves as a burning electrode and contacts the metallized foil at a point where the foil wraps around the roller, the metal tape partially embracing the roller. The tape is disposed to maintain sliding contact with the foil as the tape is wound from a first spindle to a second spindle. One of the tape (wire) spindles is electrically conductive, there being a potential difference maintained between the tape and the roller upon which the foil web is partially wrapped around. In this manner, a predetermined region of the metallized web, corresponding to the width of the tape, may be demetallized through traveling contact with the burning electrode.
Other examples of demetallizing techniques are described in Rosenberg U.S. Pat. No. 4,325,167 issued Apr. 20, 1982 and Pratt U.S. Pat. No. 3,119,919 issued Jan. 28, 1964.
However, these prior art devices and methods are limited in their ability to selectively demetallize predetermined patterns on the surface of a traveling web. The prior art apparatus tend to be incapable of demetallizing relatively fast moving webs in conjunction with relatively complex patterns. Indeed, the prior art electrical systems designed to operate on a fast moving web are typically restricted to demetallizing strips on the web which run parallel to the direction of web movement.